Fatigue Strength of Fillet Welded Joint subjected to Plate

Steel Structures 8 (2008) 163-169 www.ijoss.org

Fatigue Strength of Fillet Welded Joint subjected to Plate Bending

Biehn Baik*, Kentaro Yamada1, and Toshiyuki Ishikawa1

Department of Civil Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan1Department of Environmental Engineering and Achitecture, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

Abstract

Fatigue tests have been carried out on three types of non load-carrying fillet welded joint subjected plate bending, such assingle-side fillet welded joint, T-shaped fillet welded joint and cruciform fillet welded joint. Fatigue failure of each welded jointhas been demonstrated. The test results show that fatigue crack forms flat semi-ellipse during crack propagation and propagatesto about 80% of plate thickness before failure. The fatigue strength and life recorded under bending test have been examinedand compared with the previous results obtained by tension test. The fatigue strength of the fillet welded joint under bendingis higher than that of the welded joint under tension.

Keywords: Fatigue test, Plate bending, Fillet weld, Crack propagation

1. Introduction

Fatigue cracks developed at weld joints subjected to

bending load have received much attention. For example,

an orthotropic steel deck plate has been suffering from

fatigue cracks at the welded joints, caused by the

repeatedly flexural deformation of deck plate due to the

increase of heavy vehicle loading. The fatigue failure due

to bending stress may affect unfavorably the fatigue

performance of welded structures. Therefore, this study

has been motivated by the concern about the fatigue

behavior of welded joints subjected to plate bending.

In this study, fatigue tests are carried out on three types

of non load-carrying fillet welded joint subjected to plate

bending, such as single-side fillet welded joint, T-shaped

fillet welded joint and cruciform fillet welded joint

subjected to plate bending. Fatigue failure of each type

welded joint is examined and fatigue crack propagation

behavior is investigated by correlating crack size with a

number of cycles obtained from the fatigue test. The

fatigue strength of each type welded joint is achieved and

compared with the previous results and the design

strength specified by JSSC recommendation (1995). The

fatigue strength change due to the plate thickness increase

is discussed. In addition, to evaluate the fatigue strength

analytically, one-millimeter method (Xiao et al., 2004) is

applied.

2. Fatigue test

2.1. Test specimen

Fatigue test specimens were made from 12 mm thick

plates of JIS-SM400A. These specimens had fillet welded

rib to the structural steel plate of 12 mm thick and 300

mm wide. Details of the specimens are shown in Fig. 1.

Three types of the specimens were used. The first one

was the single-sided fillet welded joint denoted by SS.

The second one was the T-shaped fillet welded joint,

Note.-Discussion open until February 1, 2009. This manuscript forthis paper was submitted for review and possible publication on July30, 2008; approved on August 30, 2008

*Corresponding authorTel: +81-52-789-4620; Fax: +81-52-789-1674E-mail: [email protected] Figure 1. Fatigue test specimen.

164 Biehn Baik et al.

denoted by SD, and the last one was the cruciform fillet

welded joint, denoted by CR. The welding was performed

by CO2 arc procedure. The mechanical properties and

chemical compositions of the steel are given in Table 1.

2.2. Testing condition

The fatigue test was carried out using a fatigue testing

machine generating a plate bending type of loading

(Yamada et al., 2007), as shown in Fig. 2. A cantilever-

type specimen was set first on frame bed and a vibrator

was installed on the plate to generate constant amplitude

vibration. The vibration was transformed into a bending

type loading to apply to a test specimen. To control stress

ratio, R, a set of springs was adjusted to a certain desired

level. The adjustment of these springs produced tensile

stress to the specimen surface before this test was

conducted under the stress ratio, R>0.

To monitor stress ranges, strain gages were placed 5

mm away from the weld toe, as shown in Fig. 3. The

strain gages, G2 and G4 at the center of each specimen,

were selected as the applied stress range indicator. To

detect crack initiation, some copper wires of 0.04 mm in

diameter were glued on the surface of weld toe. Dye

penetrant and beach mark techniques were applied to

shape crack path on fracture surface.

3. Test results

3.1. Crack initiation and propagation

Fatigue crack location is schematically illustrated in

Fig. 4. Most specimens developed fatigue cracks at the

weld toe and failed finally. All specimens had fatigue

cracks initiated at several points of weld toe, and then

coalesced into semi-elliptical shapes during crack

propagation. Finally, the cracks propagated to about 80%

of plate thickness until failure, which is monitored by

crack initiation at the plate back surface. Some typical

fracture surfaces are shown in Fig. 5.

3.2. Crack shape variation during propagation

Crack shapes marked on the fracture surface were

measured and plotted in Fig. 6. As can be seen, as the

crack depth, a, grows, the crack length, c, grows further.

This implies that the fatigue crack growth rate tends to be

faster in surface direction than in the depth direction.

Stress gradient due to bending may delay the crack

propagation in the depth direction, while local stress

concentration due to weld accelerates crack propagation

along weld line in the surface direction.

Plotted in Fig. 7 is the variation of aspect ratio, a/c, to

depth ratio, a/t, to characterize the crack shape changes.

The test results have the decrease of a/c at the early stage

of crack propagation and then maintain the lower values

until failure, because the welded joint specimens have

small and multiple cracks coalesced together at an earlier

time before forming long and shallow semi-elliptical

cracks. This leads the rapid crack propagation in the

surface direction and then the aspect ratio drops off.

3.3. Fatigue crack propagation life

The fatigue crack propagation life of each specimen

tested is shown in Fig. 8. Since each specimen was tested

Table 1. Mechanical properties and chemical compositions of steel

Thickness(mm)

Yield strength(MPa)

Ultimate tensile strength (MPa)

Elongation(%)

Chemical compositions (%)

C Si Mn P S

12 315 431 34 0.11 0.21 0.98 0.019 0.005

Figure 3. Strain gage location.

Figure 2. Test setup. Figure 4. Fatigue crack location.

Fatigue Strength of Fillet Welded Joint subjected to Plate Bending 165

in the different stress range, the equivalent number of

cycles corresponding to the stress range, 200 MPa, was

calculated in the following form,

Neq=(∆σ/∆σe)m·n (1)

where Neq is an equivalent number of cycles, ∆σe is 200

MPa, ∆σ is an applied stress range, m is 3.0, and n is a

number of cycles obtained from the test.

Plots in Fig. 8 are comparing the measured crack sizes,

a, and c, with a number of cycles for the specimens. The

fatigue crack propagation life of SD12 is slightly shorter

than that of CR12, but less different. However, the fatigue

crack propagation life of SS12 is longer than those of

others.

Shown in Fig. 9 is a comparison of fatigue crack

propagation life associated with a type of loading, tension

and bending. Compared are the test results of CR12 and

those of the cruciform joint specimen tested under tension,

which was made of the plate with 9 mm thickness and

200 mm width and the weld leg length is 6 mm (Kim,

2000). The crack depth is normalized by the plate

thickness, and the crack length is normalized by the plate

width. Obviously, bending load contributes to the longer

fatigue life for tension. Since the stress gradient due to

bending linearly decreases in the crack depth direction,

the crack propagation is gradually slower in the depth

direction and then the fatigue crack propagation life

becomes relatively longer in bending than in tension.

The remaining fatigue life of the specimens tested

under similar stress range, 200 MPa, for tension and

bending, is shown in Fig. 10. It is seen that the remaining

fatigue life is significantly longer in bending than in

tension during crack propagation.

3.4. Fatigue strength and life

The fatigue life of failure versus the recorded nominal

stress range data are plotted on S-N curve, as shown in

Fig. 11. Also compared is the design strength for bending

specified by JSSC, which recommends that fatigue strength

for bending be equivalent to 80% of fatigue strength for

tension, if the plate thickness of welded joint is less than

25 mm. It indicates that the as-welded fillet welded joint,

such as CR12, with JSSC-E for tension, if under bending,

has JSSC-D fatigue resistance. As can be seen, the test

results exceed the design strength curve for bending.

The regression S-N lines for each set of specimens are

calculated with the inverse of the slope being set at m=3

Figure 6. Experimental crack size variation.

Figure 7. Aspect ratio, a/c and crack depth ratio, a/t.

Figure 5. Fracture surfaces.


as the following equation,

log N=c−m×log ∆σ (2)

where N is a number of cycles to failure, and ∆σ is a

nominal stress range in MPa. Results of regression

analyses for the test results are listed in Table 2. The

standard deviation s is calculated by taking log N as

variable, and the mean fatigue strength shown in the table

is the stress range at 2 million cycles to failure obtained

from regression analysis. The run-out data are not

included in the regression analysis.

As a result, the fatigue strength of SS12 was higher

than those of SD12 and CR12. The fatigue strengths at 2

million cycles of SD12 and CR12 were 28% and 22%

lower than that of SS12, respectively.

3.5. Comparison with previous test results

Several fatigue tests were carried out on welded joint

under bending in the past.

Shown in Fig. 12 is a comparison of the test results of

SS12 specimens with those of the single-sided fillet

welded specimens including the fatigue behavior of

welded joint of trough rib to an orthotropic deck plate

(Yamada et al., 2008). The specimens consisted of 6 or 8

mm thick attachment, which was fillet welded at a 78-

degree angle to the plate. Most of the specimens failed

Figure 9. Comparison of fatigue crack propagation life fortension and bending.

Figure 10. Remaining fatigue life for tension and bending.

Figure 8. Fatigue crack propagation life.


due to crack at weld root, but some failed due to crack at

weld toe. The data of toe failure are compared with the

test results of SS12. As can be seen, they show the same

fatigue strength, because both specimens have almost

same dimensions.

The test results of SD12 and the previous results

provided by Maddox (1996) and Tanaka et al. (1995) are

shown in Fig. 13. They show also about the same fatigue

strength because their geometric dimensions, for

example, plate thickness and weld size, are similar to

each other.

Shown in Fig. 14 are the test results of CR12 compared

with the previous results of the cruciform joint with K-

butt weld, which had two different plate thicknesses, 25

and 49 mm, tested by Fukuoka et al. (2006). The test

results of CR12 show about the same as those of the as-

welded specimens, in spite of the different dimension of

each specimen. The fatigue strength normally decreases

by the increase of the plate thickness in welded joint. The

JSSC specifies that the fatigue strength of welded joint

with over 25 mm thickness plate be corrected by using

the following equation.

(t≥25) (3)Ct

25 t⁄4=

Figure 11. Fatigue test results of fillet welded specimens.

Table 2. Fatigue strength at 2 million cycles

Specimentype

c sMean strength at 2×106

cycles (MPa)

SS12 13.1118 0.2806 186

SD12 12.6843 0.1548 134

CR12 12.7934 0.1653 146

Figure 12. Comparison of fatigue test results of SS12 andthe previous results.

Figure 13. Comparison of fatigue test results of SD12 andthe previous results.

Figure 14. Comparison of fatigue test results of CR12and the previous test results.

Figure 15. Fatigue strength variation due to plate thicknesschange.


where t is the plate thickness in mm.

A comparison of fatigue strengths of three-type cruciform

welded joints at 2 million cycles is shown in Fig. 15. A

dotted line stands for the design strength for tension and

a solid line represents the design strength for bending,

relating to the plate thickness change. Although the

fatigue strength tends to decrease slightly by the plate

thickness increase, its variation is less significant. The test

results are higher than the design strength corresponding

to plate thickness change and bending.

4. Fatigue Strength Evaluation byOne-millimeter Method

4.1. Finite element model

To evaluate the fatigue strength of each type welded

joint by using 1 mm method, finite element analysis

(FEA) is necessary. The stress distribution in the expected

crack path direction is usually determined by two or three

dimensional finite element analysis. The entire analysis

was carried out using the COSMOS/M 2.9. Plane strain

model with four-node two dimensional elements are used

in mesh definition for each welded joint model in this

study. An example of FE model, SD12, is shown in Fig.

16(a), with meshing around weld toe region. Minimum

mesh size in the weld toe region is 0.05 mm. In the

present analysis, Young’s modulus, E=200 GPa and Poisson’s

ratio, v=0.3 are assumed.

4.2. Fatigue life prediction by 1 mm method

The stress distribution in the expected crack path

direction is shown in Fig. 16(b). The stress at 1 mm in

depth is taken as an indicator of global geometry of

welded joint and used as a measure of fatigue strength.

The stress concentration factor at 1 mm in depth is 1.0

on the non load-carrying fillet welded cruciform joints

subjected to tension (Xiao et al., 2004). The cruciform

joints consisted of the base plate and attachment, which

were about 10 mm thick and fillet size was 6 mm. The

FEA showed that the stress concentration at 1 mm in

depth along the anticipated crack path was close to being

uniform. The scatter of test data of these cruciform joints

was defined as reference detail, and then can be referred

to determine fatigue strength of the objective welded

detail.

One-millimeter method is used to evaluate fatigue

strength of fillet welded joint subjected to bending. Stress

at 1 mm in depth is obtained for each type of specimens

from the stress distribution. Using the stress at 1 mm in

depth and the regression S-N curves of the reference

detail, fatigue strength evaluation is carried out on each

type of specimen and the result is plotted in Fig. 17 with

the fatigue test data.

The predicted life range is in good agreement with the

test data, while SS12 has rather conservative prediction.

5. Summary

The fatigue behavior of fillet welded joints subjected to

bending is the focus of this study. Fatigue test has been

carried out on three types of the fillet welded joints

subjected to bending load and crack shape development

has been examined.

The test results have indicated that most fatigue cracks

form relatively flat semi-ellipses during crack propagation

and propagate to about 80% of plate thickness before

failure. The aspect ratio, a/c, decreases at the early stage

of crack propagation and then maintain the lower values

until failure, because small and multiple cracks coalesce

at an earlier time before forming long and shallow semi-

elliptical cracks.

A comparison of fatigue crack propagation life

associated with a type of loading, tension and bending,

shows that bending obviously contributes to the longer

fatigue life than tension.

At 2 million cycles, the fatigue strength of SS12 is over

Figure 16. Example of finite element model and stress distribution.


20% higher than those of SD12 and CR12. The test

results exceed the design strength curve for bending,

specified by the JSSC, which recommends that the

fatigue strength for bending be equivalent to 80% of

fatigue strength for tension, if the plate thickness of

welded joint is less than 25 mm.

Acknowledgments

I would like to express my gratitude to Dr. S. Yamada

of Topy Industries, Ltd. and Dr. T. Ojio of Meijo

University, for their valuable advice on carrying out the

fatigue tests for bending.

References

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plate thickness and improvement by grinding on fatigue

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Figure 17. Fatigue strength evaluation by 1 mm method.

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Fatigue Strength of Fillet Welded Joint subjected to Plate